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Dermal: Gas Exchange Nervous: Sensory and Coordination Muscular: Movement Digestive: Food Intake Nutrient Absorption Reproductive: Male Female Circulatory: Gases, Nutrients, Wastes Excretory: Major Organ Systems in an Earthworm

These two slugs are showing mating behavior. The slugs are dangling on a slime thread and grip each other with their feet. The slugs evert their reproductive organs out through the gonopore. The organs unite and spermatophores are exchanged. Sperm are stored in a spermatheca for a week or more. Syngamy and deposition of zygotes occurs later.

Most Animals: Life cycle Diplontic with Oogamous Syngamy SYNGAMY zygote SporophyteGametophyte sporangium sporocyte spores gametangia gametes germination mitosis differentiation mitosis germination mitosis 1N 2N differentiation MEIOSIS

Spermato- genesis primary spermatocyte secondary spermatocyte spermatids spermatozoa meiosis I meiosis II differentiation Oogenesis primary oocyte secondary oocyte meiosis I meiosis II differentiation ootid polar bodies ovum ges/Zagreb%202007/FR2.jpg ges/Zagreb%202007/F2.jpg Compare Fig Pg. 1082

Of course canines are: polyparous (not uniparous) polytocous (not monotocous) So multiple oocytes mature in each estrus cycle. Humans are uniparous (monotocous) with some rare exceptions. We are more like sheep and cetaceans than like dogs or cats or rats. Fraternal twins appear in some families…and can have different fathers if the mother is promiscuous! Sounds like a Maury Povich show!

Which came first: the chicken or the egg? A life cycle is a circle so, this seems paradoxical to non-biologists. For biologists, the answer is an easy one! The egg evolved back when our line of evolution went from anisogamous to oogamous! This was WAY, WAY, WAY before there were any such species as a chicken (Gallus gallus)… before there were dinosaurs…before there were vertebrates…likely before there were even animals! #1

As biologists we have to agree with this cartoon:

Of course, not every “egg” is an egg… Eggs never hatch into chickens! Zygotes in shells do! What could be a “fertilized egg”?

Amphibia: Toad eggs--larval forms are strictly aquatic ORT%20PHOTOS/EX_woframplx6_se.jpg Adults are somewhat terrestrial This pair is in amplexus (mating) Syngamy is external.

Mammalia: platypus Bill of duck (but with teeth!) Fur/Hair Lays “egg” with yolk (reptile?) Syngamy is internal Milk glands feed young but no nipple, they just lick it off the fur